The DNS protocol (Cont.)
In order to allow better redundancy, each domain may have several DNS servers authoritative for its data. One will be the primary DNS server, the others will be secondary DNS servers.
Secondary DNS servers learn all info about this domain using "zone transfers" from the primary DNS server. Zone transfers are large, and thus are carried over TCP connections.
For each domain, a definition is made for how long a DNS server may cache its data before it must refresh it. This should be long enough to enhance efficiency of the protocol, but also not too long, so when changes are made to the original data, they propagate fast enough across the entire Internet.